1. Field of the Invention
The present invention relates to a semiconductor device having a circuit which includes thin film transistors (hereinbelow, abbreviated to xe2x80x9cTFTsxe2x80x9d), and a method of manufacturing the semiconductor device. More particularly, it relates to, for example, an electro-optical device which is represented by a liquid-crystal display panel, and an electronic equipment on which such an electro-optical device is installed as a component.
Incidentally, here in the specification, the expression xe2x80x9csemiconductor devicexe2x80x9d is intended to signify general devices which function by utilizing semiconductor properties, and it shall cover all of electro-optical devices, semiconductor circuits and electronic equipment.
2. Description of the Related Art
Thin film transistors (hereinbelow, abbreviated to xe2x80x9cTFTsxe2x80x9d) can be formed on a transparent substrate, and the developments of their applications to an active matrix type liquid-crystal display (hereinbelow, termed xe2x80x9cAM-LCDxe2x80x9d) have therefore been positively fostered. Since the TFTs utilizing a crystalline semiconductor film (typically, a crystalline silicon film) can attain a high mobility, it is permitted to present an image display of high definition by integrating functional circuits on an identical substrate.
In recent years, note has been taken of technology which constructs TFTs by the use of a semiconductor thin film (whose thickness is on the order of severalxe2x80x94several hundred nm) formed on a substrate having an insulating surface. The TFTs have been extensively applied to ICs and electron devices such as an electro-optical device, and the development thereof has been expedited especially as the switching elements of an image display device.
Although various applications utilizing such an image display device have been expected, applications to portable equipment have been especially noticed. Attempts have therefore been made to form TFT elements on a plastics film having a flexibility.
Since, however, the heat resistance of the plastics film is low, the highest temperature of a manufacturing process is inevitably set low. As a result, it is the present situation that TFTs of favorable electric characteristics as in the case of forming them on a glass substrate cannot be formed. Therefore, a liquid-crystal display device of high performance employing the plastics film has not been realized yet.
Besides, the AM-LCD is basically constructed so that a pixel unit for displaying an image, a gate driver circuit for driving the TFTs of individual pixels arrayed in the pixel unit, and a source driver circuit (or a data driver circuit) for transferring image signals to the respective TFTs are formed on the identical substrate.
There has recently been proposed a system-on-panel wherein, not only the pixel unit and the driver circuits, but also signal processing circuits such as a signal dividing circuit and a xcex3 correction circuit are disposed on the identical substrate.
The present invention has been made in view of the above points, and has for its object to provide an inexpensive electro-optical device. Another object of the invention is to provide an inexpensive electro-optical device which is made light in weight by employing a flexible thin substrate and forming thin film transistors on the substrate. Still another object of the invention is to provide an inexpensive electronic equipment which includes the electro-optical device as a display unit.
Further, according to the invention, improvements are made on a pixel unit. Concretely, it is also an object of the present invention to provide an electro-optical device of excellent contrast by forming a storage capacitor which can ensure a large capacity without lowering an aperture ratio.
The invention is characterized in that a substrate having a metal surface is employed as an element forming substrate (a substrate on which elements such as TFTs are formed), and that the necessary elements are formed on the substrate having the metal surface, whereby an electro-optical device is obtained. On condition that the substrate having the metal surface is thin, a semiconductor device represented by the electro-optical device, having a flexibility and being lighter in weight can be obtained.
Incidentally, the expression xe2x80x9cnecessary elementsxe2x80x9d signifies semiconductor elements (typically, TFTs) for use as the switching elements of pixels in case of an electro-optical device of active matrix type.
Besides, the invention is characterized in that a storage capacitor in a pixel unit is formed of a dielectric as which an insulating film on the substrate having the metal surface is utilized, the substrate which has the metal surface, and a drain wiring line which is connected to a semiconductor layer constituting the pixel TFT.
Concretely, according to the present invention, a semiconductor device having a substrate which has a metal surface, an insulating film which is formed on the substrate having the metal surface, and a pixel unit which is formed on the insulating film, is characterized in that said pixel unit includes a TFT, and wiring lines connected to the TFT; and that a storage capacitor is constituted by said substrate having said metal surface, said insulating film, and the wiring line.
In the above construction, the semiconductor device is characterized in that the substrate having the metal surface (called xe2x80x9cmetal substratexe2x80x9d in this specification) is a stainless steel substrate, or a substrate whose surface is coated with a metal element.
Also, in the above construction, the semiconductor device is characterized in that the substrate having the metal surface is a heat-resisting metal substrate. Besides, the semiconductor device is characterized in that the maximum height (RMAX) of the surface roughness of the substrate is 1 xcexcm or less. Further, the semiconductor device is characterized in that the radius of curvature of a convex part which exists in the surface of the substrate having the metal surface is 1 xcexcm or more.
Besides, in the above construction, the semiconductor device is characterized in that the thickness of the stainless steel substrate is 10 xcexcm-30 xcexcm.
In the above construction, the semiconductor device is characterized in that the insulating film desirably contains silicon, and that the thickness of the insulating film is 50 nm-500 nm (preferably, 50 nm-300 nm).
Further, in the above construction, the semiconductor device is characterized in that the storage capacitor is formed of the metal substrate, the insulating film, and the wiring line which is connected to a semiconductor layer constituting the pixel TFT.
Still further, in the above construction, the semiconductor device is characterized in that the wiring line is formed in contact with the insulating film and is connected to a pixel electrode.
Meanwhile, a method of manufacturing the above semiconductor device according to the invention is characterized by comprising the step of forming a first insulating film on a substrate which has a metal surface: the step of forming a semiconductor layer on said first insulating film; the step of forming a second insulating film on said semiconductor layer; the step of forming a gate electrode on said second insulating film; the step of forming a third insulating film so as to cover said semiconductor layer and said gate electrode; the step of partly removing said third insulating film, thereby to expose part of said semiconductor layer and part of said first insulating film; and the step of forming a wiring line which is electrically connected to said semiconductor layer, and which lies in contact with said part of said first insulating film.
In the above construction, the manufacturing method is characterized in that the wiring line is formed in connection with the parts of the semiconductor layer and the first insulating film.
Also, in the above construction, the manufacturing method is characterized in that a storage capacitor constituting a pixel unit is formed of the metal substrate, the part of the first insulating film, and the wiring line.
Besides, in the above construction, as the first insulating film is thinner, the capacity of the storage capacitor can be enlarged more. Moreover, in a case where the contact area between the first insulating film and the wiring line is larger, the capacity can be enlarged more.